1. Field of the Invention
This invention relates to steam turbines and the devices used to stop large particles carried by the inlet steam from entering the turbine.
2. Description of the Prior Art
All steam turbines include strainers to capture particles carried by steam from the boiler and to prevent the particles from entering the turbine where serious damage could occur.
Existing turbines use one of two general types of strainers.
The first type is a cylinder with large holes that are arranged to permit a screen with smaller openings to be wrapped around the cylinder, thus having the ability to stop steam carried particles larger than approximately 1/8 inch in size. This same system can also permit the temporary addition of a finer mesh screen for brief operating periods when the presence of particles from the boiler is known to be a serious threat. This type of strainer presents a minor deficiency in that the screen itself may fail with time and operating use thus causing internal turbine damage of a serious nature. The steam pressure drop is also increased by the necessity of passing through both a screen and the strainer holes.
The second type of strainer is a simple cylinder with many small holes. The holes allow the passage of steam, but stop particles larger than the hole size selected. The holes selected vary from one turbine design to another, with some designers selecting approximately 1/4 inch holes; while others use approximately 3/32 inch holes. This type strainer uses holes with square,sharp inlets, such that the flow coefficient is relatively small, causing a greater pressure drop of the steam entering the turbine. These holes are further vulnerable to inlet damage caused by high velocity particles which further reduces the effective flow area and increases the undesirable steam pressure drop.